1. Field of the Invention
This invention relates to an optoelectronic device for scanning videofrequency images in which provision is made for correction of thermal drift.
2. Description of the Prior Art
An optoelectronic videofrequency-image scanning device was disclosed in French Pat. No. 2,363,084 published on Mar. 24th, 1978. This known device comprises optical receiving means, optical elements driven in such a manner as to produce an image scan, an optical element for focusing on a detection system comprising an array of detectors and videofrequency means for processing the signals delivered by the detectors. The optoelectronic device for scanning videofrequency images is of the line-by-line type.
At the end of each image scan, a device such as a pattern, for example, is placed in front of the detectors in order to produce a uniform luminous flux on all the detectors. By making use of a correction loop, the signals of each detector can thus be corrected for each image scan by subtracting, in the case of each detector, the memorized value corresponding to the detected uniform flux. This correcting unit, in which a correction loop and a pattern are associated, makes use of preamplifiers provided with an input capacitor for retaining the direct-current components of the signals since recalibration takes place at each image. As a result of periodic recalibration, each measuring chain can be recalibrated prior to each image scan, thus making it possible to take into account the elimination of direct-current components from the signals and to compensate for thermal drift, especially of the first stage of amplification of the signals of each detector without, however, impairing restitution of the image. At the time of an image scan, the time constants of the preamplifier inputs are accordingly chosen so as to ensure that direct-current signals are not reduced in strength by more than 1% during one image-scanning period. The disadvantage attached to the use of a correcting unit of this type lies in the fact that it occupies a substantial proportion of the processing time (at least 25%) solely for recalibrating operations which must essentially be performed at each image scan. Furthermore, and as will become apparent hereinafter, the use of a correcting unit of this type in an optoelectronic device for scanning of images by rotary sweeping would also entail the loss of part of the image, especially in the central zone.